Apnea of prematurity and the Sudden Infant Death Syndrome are both important clinical problems related to apnea. They may be best approached through basic research into the physiological basis of apnea. We have recently shown that apnea in newborn infants occurs at the minimum phase of oscillatory breathing patterns rather than being an isolated or random event. In our new studies we want to know the physiological basis for these patterns, as well as the limits of the apnea - breathing pattern relationship. Our studies also will yield basic information about the dynamics of respiratory control in infants. The research plan includes two areas of investigation. The first area involves three studies into the causes and consequences of oscillatory breathing patterns in premature infants. First, oscillatory patterns will be accentuated by sinusoidal variation of inspired C02. The maximum duration of apnea associated with these accentuated patterns will then be compared to our observed endogenous apnea limit, which is apnea duration less than or equal to one-half of the pattern cycle time. Second, the hypothesis that the patterns are due to instability in the respiratory control system as opposed to a neurological oscillator driving respiration will be tested by controlling end tidal PC02 and P02 independently from ventilation, thus opening the control loop. Patterns seen when the control loop is open cannot be due to instability in that control loop. Third, a rigorous stability analysis of the infant's respiratory control system will be performed using sinusoidal variation of end tidal PC02. The second area involves mathematical modeling of the infant's respiratory control system. Although many models have been offered to explain periodic breathing phenomena in adults, when they are scaled to represent infants they do not produce patterns compatible with those we have seen in infants. Our modeling efforts will be based on our own preliminary model and on Khoo's linearized perturbation model.